U.S. Pat. No. 3,896,131 broadly describes 6,7-dihydro-5-methyl-1-oxo-1H,5H-benzo[ij]quinolizine-2-carboxylic acids as active antimicrobial agents. The compound 10-cyano-6,7-dihydro-5-methyl-1-oxo-1H,5H-benzo[ij]quinolizine-2-carboxyli c acid is specifically disclosed in the aforementioned patent (Example 71). It has now been found that the corresponding novel 8-cyano compound is much more potent and has a broader spectrum of antimicrobial activity than the 10-cyano compound. Accordingly, the 8-cyano compounds of the present invention are much safer and more economical to use than the 10-cyano compound of the prior art.